Test the coolant temperature sensors near the radiator and thermostat housing with a service manual and ohmmeter; the connectors usually lock and rarely just pop off. A later shop diagnosis found the sensor under the distributor bad, which may require distributor removal to replace.
The owner of a 1999 Civic reported an inoperative temperature gauge and suspected a loose wire. The expert advised checking the sensors by the radiator and thermostat housing with a manual and ohmmeter rather than assuming a connector came off. The owner later said a shop found the sensor under the distributor bad and also reported a bad VTEC solenoid and VTEC oil pressure switch, then asked where to find VTEC parts.
